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Summary: Inherited Constants | Ctors | Methods | Inherited Methods

# RuleBasedCollator

`public class RuleBasedCollator`
`` `extends Collator` ``

java.lang.Object
↳ | java.text.Collator
| ↳ | java.text.RuleBasedCollator

The `RuleBasedCollator` class is a concrete subclass of `Collator` that provides a simple, data-driven, table collator. With this class you can create a customized table-based `Collator`. `RuleBasedCollator` maps characters to sort keys.

`RuleBasedCollator` has the following restrictions for efficiency (other subclasses may be used for more complex languages) :

1. If a special collation rule controlled by a <modifier> is specified it applies to the whole collator object.
2. All non-mentioned characters are at the end of the collation order.

The collation table is composed of a list of collation rules, where each rule is of one of three forms:

        <modifier>
        <relation> <text-argument>
        <reset> <text-argument>

The definitions of the rule elements is as follows:

- Text-Argument: A text-argument is any sequence of characters, excluding special characters (that is, common whitespace characters [0009-000D, 0020] and rule syntax characters [0021-002F, 003A-0040, 005B-0060, 007B-007E]). If those characters are desired, you can put them in single quotes (e.g. ampersand => '&'). Note that unquoted white space characters are ignored; e.g. `b c` is treated as `bc`.
- Modifier: There are currently two modifiers that turn on special collation rules.
  - '@' : Turns on backwards sorting of accents (secondary differences), as in French.
  - '!' : Turns on Thai/Lao vowel-consonant swapping. If this rule is in force when a Thai vowel of the range \U0E40-\U0E44 precedes a Thai consonant of the range \U0E01-\U0E2E OR a Lao vowel of the range \U0EC0-\U0EC4 precedes a Lao consonant of the range \U0E81-\U0EAE then the vowel is placed after the consonant for collation purposes.

  '@' : Indicates that accents are sorted backwards, as in French.

- Relation: The relations are the following:
  - '<' : Greater, as a letter difference (primary)
  - ';' : Greater, as an accent difference (secondary)
  - ',' : Greater, as a case difference (tertiary)
  - '=' : Equal
- Reset: There is a single reset which is used primarily for contractions and expansions, but which can also be used to add a modification at the end of a set of rules.

  '&' : Indicates that the next rule follows the position to where the reset text-argument would be sorted.

This sounds more complicated than it is in practice. For example, the following are equivalent ways of expressing the same thing:

>      a < b < c
>      a < b & b < c
>      a < c & a < b

Notice that the order is important, as the subsequent item goes immediately after the text-argument. The following are not equivalent:

>      a < b & a < c
>      a < c & a < b

Either the text-argument must already be present in the sequence, or some initial substring of the text-argument must be present. (e.g. "a < b & ae < e" is valid since "a" is present in the sequence before "ae" is reset). In this latter case, "ae" is not entered and treated as a single character; instead, "e" is sorted as if it were expanded to two characters: "a" followed by an "e". This difference appears in natural languages: in traditional Spanish "ch" is treated as though it contracts to a single character (expressed as "c < ch < d"), while in traditional German a-umlaut is treated as though it expanded to two characters (expressed as "a,A < b,B ... &ae;\u00e3&AE;\u00c3"). [\u00e3 and \u00c3 are, of course, the escape sequences for a-umlaut.]

Ignorable Characters

For ignorable characters, the first rule must start with a relation (the examples we have used above are really fragments; "a < b" really should be "< a < b"). If, however, the first relation is not "<", then all the all text-arguments up to the first "<" are ignorable. For example, ", - < a < b" makes "-" an ignorable character, as we saw earlier in the word "black-birds". In the samples for different languages, you see that most accents are ignorable.

Normalization and Accents

`RuleBasedCollator` automatically processes its rule table to include both pre-composed and combining-character versions of accented characters. Even if the provided rule string contains only base characters and separate combining accent characters, the pre-composed accented characters matching all canonical combinations of characters from the rule string will be entered in the table.

This allows you to use a RuleBasedCollator to compare accented strings even when the collator is set to NO_DECOMPOSITION. There are two caveats, however. First, if the strings to be collated contain combining sequences that may not be in canonical order, you should set the collator to CANONICAL_DECOMPOSITION or FULL_DECOMPOSITION to enable sorting of combining sequences. Second, if the strings contain characters with compatibility decompositions (such as full-width and half-width forms), you must use FULL_DECOMPOSITION, since the rule tables only include canonical mappings.

Errors

The following are errors:

- A text-argument contains unquoted punctuation symbols (e.g. "a < b-c < d").
- A relation or reset character not followed by a text-argument (e.g. "a < ,b").
- A reset where the text-argument (or an initial substring of the text-argument) is not already in the sequence. (e.g. "a < b & e < f")

If you produce one of these errors, a `RuleBasedCollator` throws a `ParseException`.

Examples

Simple: "< a < b < c < d"

Norwegian: "< a, A < b, B < c, C < d, D < e, E < f, F < g, G < h, H < i, I < j, J < k, K < l, L < m, M < n, N < o, O < p, P < q, Q < r, R < s, S < t, T < u, U < v, V < w, W < x, X < y, Y < z, Z < \u00E6, \u00C6 < \u00F8, \u00D8 < \u00E5 = a\u030A, \u00C5 = A\u030A; aa, AA"

To create a `RuleBasedCollator` object with specialized rules tailored to your needs, you construct the `RuleBasedCollator` with the rules contained in a `String` object. For example:

>      String simple = "< a< b< c< d";
>      RuleBasedCollator mySimple = new RuleBasedCollator(simple);

Or:

>      String Norwegian = "< a, A < b, B < c, C < d, D < e, E < f, F < g, G < h, H < i, I" +
>                         "< j, J < k, K < l, L < m, M < n, N < o, O < p, P < q, Q < r, R" +
>                         "< s, S < t, T < u, U < v, V < w, W < x, X < y, Y < z, Z" +
>                         "< \u00E6, \u00C6" +     // Latin letter ae & AE
>                         "< \u00F8, \u00D8" +     // Latin letter o & O with stroke
>                         "< \u00E5 = a\u030A," +  // Latin letter a with ring above
>                         "  \u00C5 = A\u030A;" +  // Latin letter A with ring above
>                         "  aa, AA";
>      RuleBasedCollator myNorwegian = new RuleBasedCollator(Norwegian);

A new collation rules string can be created by concatenating rules strings. For example, the rules returned by `getRules()` could be concatenated to combine multiple `RuleBasedCollator`s.

The following example demonstrates how to change the order of non-spacing accents,

>      // old rule
>      String oldRules = "=\u0301;\u0300;\u0302;\u0308"    // main accents
>                      + ";\u0327;\u0303;\u0304;\u0305"    // main accents
>                      + ";\u0306;\u0307;\u0309;\u030A"    // main accents
>                      + ";\u030B;\u030C;\u030D;\u030E"    // main accents
>                      + ";\u030F;\u0310;\u0311;\u0312"    // main accents
>                      + "< a , A ; ae, AE ; \u00e6 , \u00c6"
>                      + "< b , B < c, C < e, E & C < d, D";
>      // change the order of accent characters
>      String addOn = "& \u0300 ; \u0308 ; \u0302";
>      RuleBasedCollator myCollator = new RuleBasedCollator(oldRules + addOn);

See also:

- `Collator`
- `CollationElementIterator`

## Summary

### Inherited constants
From class `java.text.Collator`
`int` | `CANONICAL_DECOMPOSITION` Decomposition mode value.
`int` | `FULL_DECOMPOSITION` Decomposition mode value.
`int` | `IDENTICAL` Collator strength value.
`int` | `NO_DECOMPOSITION` Decomposition mode value.
`int` | `PRIMARY` Collator strength value.
`int` | `SECONDARY` Collator strength value.
`int` | `TERTIARY` Collator strength value.

### Public constructors
`RuleBasedCollator(String rules)` RuleBasedCollator constructor.

### Public methods
`Object` | `clone()` Standard override; no change in semantics.
`int` | `compare(String source, String target)` Compares the character data stored in two different strings based on the collation rules.
`boolean` | `equals(Object obj)` Compares the equality of two collation objects.
`CollationElementIterator` | `getCollationElementIterator(String source)` Returns a CollationElementIterator for the given String.
`CollationElementIterator` | `getCollationElementIterator(CharacterIterator source)` Returns a CollationElementIterator for the given CharacterIterator.
`CollationKey` | `getCollationKey(String source)` Transforms the string into a series of characters that can be compared with CollationKey.compareTo.
`String` | `getRules()` Gets the table-based rules for the collation object.
`int` | `hashCode()` Generates the hash code for the table-based collation object

### Inherited methods
From class `java.text.Collator`
`Object` | `clone()` Returns a new collator with the same decomposition mode and strength value as this collator.
`int` | `compare(Object o1, Object o2)` Compares its two arguments for order.
`abstract int` | `compare(String source, String target)` Compares the source string to the target string according to the collation rules for this Collator.
`boolean` | `equals(String source, String target)` Convenience method for comparing the equality of two strings based on this Collator's collation rules.
`boolean` | `equals(Object that)` Compares the equality of two Collators.
`static Locale[]` | `getAvailableLocales()` Returns an array of all locales for which the `getInstance` methods of this class can return localized instances.
`abstract CollationKey` | `getCollationKey(String source)` Transforms the String into a series of bits that can be compared bitwise to other CollationKeys.
`int` | `getDecomposition()` Get the decomposition mode of this Collator.
`static Collator` | `getInstance()` Gets the Collator for the current default locale.
`static Collator` | `getInstance(Locale desiredLocale)` Gets the Collator for the desired locale.
`int` | `getStrength()` Returns this Collator's strength property.
`abstract int` | `hashCode()` Generates the hash code for this Collator.
`void` | `setDecomposition(int decompositionMode)` Set the decomposition mode of this Collator.
`void` | `setStrength(int newStrength)` Sets this Collator's strength property.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.util.Comparator`
`abstract int` | `compare(Object o1, Object o2)` Compares its two arguments for order.
`static <T, U> Comparator<Object>` | `comparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)` Accepts a function that extracts a sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key using the specified `Comparator`.
`static <T, U extends Comparable<? super U>> Comparator<Object>` | `comparing(Function<? super T, ? extends U> keyExtractor)` Accepts a function that extracts a `Comparable` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
`static <T> Comparator<Object>` | `comparingDouble(ToDoubleFunction<? super T> keyExtractor)` Accepts a function that extracts a `double` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
`static <T> Comparator<Object>` | `comparingInt(ToIntFunction<? super T> keyExtractor)` Accepts a function that extracts an `int` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
`static <T> Comparator<Object>` | `comparingLong(ToLongFunction<? super T> keyExtractor)` Accepts a function that extracts a `long` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
`abstract boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this comparator.
`static <T extends Comparable<? super T>> Comparator<Object>` | `naturalOrder()` Returns a comparator that compares `Comparable` objects in natural order.
`static <T> Comparator<Object>` | `nullsFirst(Comparator<? super T> comparator)` Returns a null-friendly comparator that considers `null` to be less than non-null.
`static <T> Comparator<Object>` | `nullsLast(Comparator<? super T> comparator)` Returns a null-friendly comparator that considers `null` to be greater than non-null.
`static <T extends Comparable<? super T>> Comparator<Object>` | `reverseOrder()` Returns a comparator that imposes the reverse of the natural ordering.
`default Comparator<Object>` | `reversed()` Returns a comparator that imposes the reverse ordering of this comparator.
`default <U extends Comparable<? super U>> Comparator<Object>` | `thenComparing(Function<? super T, ? extends U> keyExtractor)` Returns a lexicographic-order comparator with a function that extracts a `Comparable` sort key.
`default <U> Comparator<Object>` | `thenComparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)` Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given `Comparator`.
`default Comparator<Object>` | `thenComparing(Comparator<? super T> other)` Returns a lexicographic-order comparator with another comparator.
`default Comparator<Object>` | `thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)` Returns a lexicographic-order comparator with a function that extracts a `double` sort key.
`default Comparator<Object>` | `thenComparingInt(ToIntFunction<? super T> keyExtractor)` Returns a lexicographic-order comparator with a function that extracts a `int` sort key.
`default Comparator<Object>` | `thenComparingLong(ToLongFunction<? super T> keyExtractor)` Returns a lexicographic-order comparator with a function that extracts a `long` sort key.

## Public constructors

### RuleBasedCollator

    public RuleBasedCollator (String rules)

RuleBasedCollator constructor. This takes the table rules and builds a collation table out of them. Please see RuleBasedCollator class description for more details on the collation rule syntax.

Parameters
`rules` | `String`: the collation rules to build the collation table from.

Throws
`ParseException` | A format exception will be thrown if the build process of the rules fails. For example, build rule "a < ? < d" will cause the constructor to throw the ParseException because the '?' is not quoted.

See also:

- `Locale`

## Public methods

### clone

    public Object clone ()

Standard override; no change in semantics.

Returns
`Object` | a shallow copy of this collator.

### compare

    public int compare (String source,
                    String target)

Compares the character data stored in two different strings based on the collation rules. Returns information about whether a string is less than, greater than or equal to another string in a language. This can be overriden in a subclass.

Parameters
`source` | `String`: the source string.
`target` | `String`: the target string.

Returns
`int` | Returns an integer value. Value is less than zero if source is less than target, value is zero if source and target are equal, value is greater than zero if source is greater than target.

Throws
`NullPointerException` | if `source` or `target` is null.

### equals

    public boolean equals (Object obj)

Compares the equality of two collation objects.

Parameters
`obj` | `Object`: the table-based collation object to be compared with this.

Returns
`boolean` | true if the current table-based collation object is the same as the table-based collation object obj; false otherwise.

### getCollationElementIterator

    public CollationElementIterator getCollationElementIterator (String source)

Returns a CollationElementIterator for the given String.

Parameters
`source` | `String`: the string to be collated

Returns
`CollationElementIterator` | a `CollationElementIterator` object

See also:

- `CollationElementIterator`

### getCollationElementIterator

    public CollationElementIterator getCollationElementIterator (CharacterIterator source)

Returns a CollationElementIterator for the given CharacterIterator.

Parameters
`source` | `CharacterIterator`: the character iterator to be collated

Returns
`CollationElementIterator` | a `CollationElementIterator` object

See also:

- `CollationElementIterator`

### getCollationKey

    public CollationKey getCollationKey (String source)

Transforms the string into a series of characters that can be compared with CollationKey.compareTo. This overrides java.text.Collator.getCollationKey. It can be overriden in a subclass.

Parameters
`source` | `String`: the string to be transformed into a collation key.

Returns
`CollationKey` | the CollationKey for the given String based on this Collator's collation rules. If the source String is null, a null CollationKey is returned.

### getRules

    public String getRules ()

Gets the table-based rules for the collation object.

On Android, the returned string will be empty unless this instance was constructed using `RuleBasedCollator(java.lang.String)`.

Returns
`String` | returns the collation rules that the table collation object was created from.

### hashCode

    public int hashCode ()

Generates the hash code for the table-based collation object

Returns
`int` | a hash code value for this object.

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